Piston and Rod Repair
This repair procedure requires removeal of the cylinder head and oil pan.It is much easier to perform this work with the engine removed from the vehicle and mounted on a stand.These procedures require certain hand tools.A cylinder ridge reamer,a numbered punch set,piston ring expander,snapring tools and piston installment tool(ring compressor) are necessary for correct piston and rod repair.
Cleaning&inspection
If the piston is out of specification or otherwise unusable,it must be replaced.If the cylinder is worn or damaged ,the engine block must be bored and oversize pistons intalled.
1.Remove the piston rings from the piston .The compression rings(top and middle rings)must be removed using a piston ring expander,to prevent breakage and/or scatching the piston.
2.If there is no obvious damage to the piston and the intent is to resue the piston ,clean the ring grooves with a ring groove cleaner,being careful not to cut into the piston metal.
Heavy carbon deposits can be cleaned from the top of piston with a scraper or wire brush.Do not ,however,use a wire wheel on the ring groove or lands.Clean the oil drain holes in the ring grooves.Clean all remainingdirt,carbon and varnish from the piston with a suitable solvent and a brush.Do not use a caustic solution on alumium parts.
3.After cleaning ,inspect the piston for scuffing,scoring,cracks,pitting or excessive ring groove wear.Replace the piston if obviously worn.
4.If the piston appears serviceable,meature the piston diameter using a micrometer.Measure the piston diameter in the thrust direction,90 degrees to the pistonpin axis,1-1/4 inch below the top of the piston.
5.Inspect the cylinder bores for taper and out-of-round.The cylinder bores must be measured at 3 levels top to bottom in directions of East-to-West and North-to-South.Measure the cylinder diameter using a bore gauge,or with a telescope gauge and micrometer.The measurement should be made in the piston thrust direction at the top,middle and bottom of the cylinder .Note:piston diameter and cylinder measurement should be made with the parts at room temperature,70°F(21℃) .
6.Subtract the piston diameter measurement from the cylinder measurement.This is the piston-to-cylinder wall clearance.If the clearance is excessive or if the cylinder wall is badly scored or scuffed,the cylinder may have to be bored and an oversize piston installed.
7.Vertify that the cylinder has a proper cross-hatch pattern.These tiny marksare the result of the honing operation during engine manufacture.They retain engine oil to keep thepiston rings from scuffing during engine break-in after overhaul.If little or no good condition or rebore if the cylinder is worn or damaged.
8.If the piston-to-cylinder wall clearance is within specifications,checkthe ring groove clearance.Roll the piston ring around the ring groove in which it is to be installed and sheck the clearance with a feeler gauge.Compare the measurememt with specification.High points in the ring groove that may cause the ring to bind may be cleaned up carefully with a small flat file.Raplace the piston if the ting groove clearance is not within specification.
9.Check the connecting rod for damage or obvious wear.Check for signs of over-heating(blue appearance) or fractures and check the bearing bore for out-of-round and taper.Ashiny surface on the pin boss side of the piston usually indicates that the connecting rod os bent or the wrist pin hole is not in proper relation to the piston skirt and ring grooves.Abnormal connecting rod bearing wear can be causes by either a bent connecting rod, an improperly machined journal or a tapered connecting rod bore.Twisted connecting rods will disturb the action of the entire piston,rings,and connecting rod assembly and may be the cause of excessive oil consumption.
10.If the piston must be removed from the connecting rod,mark the side of the connecting rod that coresponds with the side of the the piston that faces the front of the engine,so the new piston will be installed facing the same direction.Most piston have an arrow or notch on the top of the piston,indicating that this side should face the front of the engine.If the original piston is to be reinstalled,use paint or a marker to indicate the cylinder number on the piston,so it can be reinstalled on the sane connecting rod.
11.T he piston pin is a press fit in the connecting rod.If the piston and/or connecting rod must be replaed ,the pin must be pressed into the connecting rod using a fixture that will not damage or distort the piston and/or connecting rod.The piston must move freely on the pin after installation.
Ridge Removal&Honing
Ridge removal
This particular procedure applies to all vehicles.Inspect the upper portions of the cylinder(near the head)for a ridge formed by ring wear.If there is a ridge,it must be removed by first shifting the piston down in the cylinder and then covering the piston top completely with a clean rag.Use a ridge reamer to remove metal at the lip until the cylinder is smooth.If this is not done, the rings will be damaged during removal of the piston.
Honing
Before honing the cylinders,stuff plenty of clean shop towls under the bores and over the crankshaft(if still in the engine) to keep the abrasive materials from entering the crankcase area.
1.The cylinder bore reisizing hone equipped with 220 grit stones or eqivalent is the best tool for this job.In addition to deglazing,it will reduce taper and out-of-round as well as removing light scuffing,scoring or scratches.Usually a few strkes will clean up a bore and maintain the required limits.
2.Deglazing of the cylinder walls may be done using a cylinder surfacing hone(or equivalent).Use a tool equpped with 280 grit stones if the cylinder is alredy straight and round.Usually 20-60 strokes,depending on the bore condition will be sufficient to provide a satisfactory surface.Inspect th ecylinder walls after each 20 strokes,using a light honing oil available from an automotive parts store.
3.Honing should be done by moving the hone up and down fast enough to get a cross-hatch pattern.When hone marks intersect at 50-60°,the cross-hatch angle is most satisfactory for proper seating of the rings.
4.A controlled hone motor speed between 200-300rpm is necessary to obtain the proper cross-hatch angle.The number of up and down strokes per minite can be regulated to get the desired 50-60° angle.Faster up and down strokes increase the cross-hatch angle.
5.After honing,it will be necessary to clean the block to remove all traces of obrasive.
WARNING:Be sure that all abrasive is removed from the engine parts after honing. It is recommended that a solution of soap and hot water be usesd with a brush and the parts then thoroughly dried.The bore is considered clean when it can be wiped with a white cloth and the cloth remains clean.Oil the bores after xleaning to prevent rust.
Piston pin replacement
The piston pins are press-fit into the piston/connecting rod assemblies.The piston pin must be heated up to an extreme temperature often as high as 1500°F(815℃)or higher,and then the pin must be pressed into the piston and connecting rod with a special press.Therefore ,because of the special machanery and specific skills needed to either remove the old piston pin or to install the new piston pin,the piston,connecting rod and piston pin should be taken to a qualified machine shop.
Piston ring replacement
1.Wipe the cylinder bore clean. By hand,gently compress the piston rings to be used in the cylinder,one at a time,and using an incerted piston to keep the ring square to the cylinder bore,push the piston ring down into the cylinder bore.
Using a feeler gauge,check the piston ring gap with the ring positioned at least 1.50 inch(12mm)from the bottom of the cylinder bore.Make sure the measurement is within specifications A gap that is too tight is more harmful than one that is too loose.If the ring end gap is excessively loose, the cylinder is probably worn beyond specification.
2.If the ring end gap is too tight,carefully remove the ring and file the endssquarely with a fine file to abtain the proper cleanrance.
If the instruction on the ring parkaging differ from the following piston ring installation information including ring gap postioning,follow the ring manufacturer’s instructions.
3.Install the rings on the piston, lowest ring first.The lowest or bottom groover ring is the oil control ring and is a multi-piece ring set consisting of an upper and lower rail and a center expander ring.These pieces are installed by hand, do not use a piston ring and expander in the bottom ring groove.This expander goes on easily since it has very little tension.Make sure the ends butt together and do not overlap.Start one end od an oil ring rail ring into the oil ring groove above the expander,hold the end firmly and push down the portion to be installed until it is in position.Finish installing the rail ring by spiraling it the remainder of the way on.Repeat the rail installation with the other rail ring.Pay attention to the location of the ring gaps in relation to the piston circumference.On the oil ring rails,the gaps must be 180 degree apart.
4.The upper and intermediate piston rings mey have a different cross section.Use care to select the proper ring.Install the piston rings with the manufacturer’s identification mark facing UP.The piston sing packaging should contain instrctions as to the directions the ting side should face.The top 2 rings must be installed using a piston ring to expander tool.There is a high risk of breaking or distorting the compression rings if they are installed by hand.Spread the intermadiate ring carefully with the expander tool and install on the piston.Repea this step to install the top copression ring using the piston ring expander toll.Stagger the ring end gaps by approximately 120 degrees.The compression ring end gaps must not be aligned,nor should the upper teo piston ring gaps align with the oil control rail gaps.Staggered ring gaps are important for oil control.
Rod bearing replacement
The engine crankshaft and connecting rod bearing clearances can be determined by the use of Plastigage or a similar product.The following is the recommended procedure for the use of Plastigage:
1.Rotate the crankshaft until the connecting rod boss to be check ed is at the bottom of its stroke.
2.With the connecting rod and the connectingrod bearing shell in the bearing cap removed from the crankshaft,remove the oil film from the surface to be checked.Plastigage is soluble in oil.
3.Place a piece of Plastigage across the entire width of the bearing shell in the bearing cap approximately 1/4 inch off center and away from the oil hole.In addition,suspect area can be checked by placing Plastigage in the suspect area.
4.Before assembling the rod bearing cap with the Plastigage in place,the crankshaft must be rotated until the connecting rod being checked starts moving toward the top of the engine.Only then should the cap be assembled and tightened to specifications.
Do not rotate the crankshaft while assemnling the cap or Plastigage may be ameared, giving inaccurate results.
5.Remove the bearing cap and compare the width of the flattened Plastigage with the metric scale provided on the package.Locate the band closest to the same width.This band shows the amount of clearance in thousandths of a millimeter.Differences in readings between the ends indicate the amount of taper present.Record all readings taken.Plastigage generally is accompanied by two scales.One scale is in inches,the other is a metric scale,Plastigage is available in a variety of clearance ranges.The 0.001-0.003 inch is usually the most appropriate for checking engine bearing proper specifications
auther:kelly www.sinodiesel.com
